Director of the Dresden Gemäldegalerie (1913-1942) and Nazi administrator in charge of the depository of looted art during the Reich years. Posse's father was a respected archivist in Dresden. The younger Posse joined the Dresden gallery, the state gallery of Saxony in 1910 and became director in 1913. He issued an updated edition of the Dresden holdings catalog of Karl Woermann. From the first as director, he showed a special callousness toward returning loaned pictures. During World War I, Raphael's "Portrait of a Youth" (Czartoryski Collection of the National Gallery, Warsaw, Poland) was moved to Dresden for safekeeping. After the war, Posse delayed returning the picture until 1920. When the Nazi's gained power in Germany, Posse was briefly removed from office for anti-Nazi sentiments and having acquired art they considered "degenerate," i.e., modern. He was reinstated by Hitler personally in 1939 and placed in charge of creating a museum in Linz, Austria, for the Führer. The project was referred to as the "Sonderauftrag" (special commission). The same year, Posse confiscated the collection of the Rothschild brothers, Alphonse Mayer Rothschild (1878-1942) and Louis Rothschild (1882-1955). Assisted by the Kunsthistorisches director, Ludwig von Baldass, plunderers took thousands of objects from the long-time Jewish Vienese residents. The treasures were taken to the Hofburg palace, the Nazis' Zentraldepot for confiscated art. Lesser pieces went to Viennese museums, including the Kunsthistorisches, while much of the decorative arts, mainly porcelains, were auctioned at the state-owned Dorotheum. Posse was aided in his art thefts by the unscrupulous Berlin dealer Karl Haberstock (1878-1956). After the invasion of Holland, Posse moved to the The Hague as Referent für Sonderfragen (Adviser on "Special Questions"). With the aid of the Seyss-Inquart government, Posse's office forced the purchase of major portions of the collection of Fritz Mannheimer (1890-1939) in 1944, including "The Jewish Doctor" by Rembrandt. The Fürherbefehl directive authorized Posse to "make decisions on the Führer's behalf" for works of art discovered throughout the Reich. By 1941, Posse's mission was aligned with the Einsatzstab Reichsleiter Rosenberg (ERR) confiscations. Posse acquired Vermeer's "The Artist in His Studio," then in the hands of the Czernin family of Vienna, in1940, through the intervention of Reichsleiter Baldur von Schirach (1907-1974) after accusations of unpaid taxes to the family failed. Posse added three other Rembrandts to the Linz Collection, in addition to Watteau's "La Danse" from Potsdam, the Corsini Memling portrait, and Rubens' "Ganymede." 21,000 objects were seized in France alone, of which the Linz collection took all but the 700--the others earmarked for the collection of Hermann Göring, another Nazi official collecting stolen art. Other specialists who advise Posse included Friedrich Wolffhardt (1899-?), a captain in the SS, who handled rare books and manuscripts; Fritz Dworschak, an Austrian Nazi party loyal, in charge of coins and medal collections; and Hans Reger, an architect from Munich to document the objects in photographs. Posse also hired a personal assistant, Robert Oertel, an Italian paintings scholar, to keep the records. Posse contracted cancer of the tongue and died in 1942. His funeral was attended by many ranking Nazis; Joseph Goebbels (1897-1945), the minister of propaganda, read the eulogy. He was succeeded, both at the Dresden Gemäldegalerie and the Führermuseum in 1943 by Hermann Voss. As the Allies moved into Europe in 1944, the art was moved to the Altaussee salt mines near Salzburg for safe keeping. After capture in 1945, the American 42nd Division army authorized the Austrian government to handle restitution. However, the Rothschilds were coerced to leave many works already in Austrian museums, including the eleven paintings to Kunsthistorisches, thirty works on paper to the Albertina, forty-three to the Museum of Applied Art, and eight works to the Belvedere Gallery. The Austrian government only returned works to the Rothschild heirs in 1999.

Despite his complicity in assembling a Nazi art collection (Posse acquired over
2500 objects in three years), Posse disagreed so strongly with the Nazi view of
modern art that it forced him into early retirement. As a
scholar, he updated several excellent catalogs for the Dresden gallery.